The object of this proposed research is to determine the details of the biosynthetic pathway leading to th formation of the 5,6-dimethylbenzimidazole moiety of Vitamin B12 in Propionibacterium. Our efforts are concentrated in two areas. We are investigating the origin of the four-carbon biosynthetic unit involved in dimethylribityllumazine formation and we are also planning to iientify, isolate, and characterize the enzymes involved in the biosnthetic conversion of riboflavin into dimethylbenzimidazole.